Saturday, December 22, 2012

Unresolved issues in the latest round of DNA testing in the Hank Skinner case

DNA testing in the Hank Skinner case began again in 2012 after the state of Texas dropped its objections after a fight that lasted over a decade.  After the release of some preliminary data, the prosecution declared that it confirmed Skinner’s guilt, as reported by Jordan Smith in the Austin Chronicle.  Mr. Skinner’s attorney issued a response stating that not all testing was complete and that defense experts had not had a chance to examine the results yet.  Skinner had been found guilty of murdering his live-in girlfriend Twila Busby and her two grown sons.  He claims that he is innocent, and there is an alternate suspect in this case, Robert Donnell, who was Twila’s uncle.

Mr. Skinner’s DNA was found on 10 of 40 items tested.  Many of the items tested appear to have been blood, and they come from various locations around the house.  There are profiles that belong to an unknown individual or individuals, including one unknown profile on a knife that has DNA deposited by Mr. Skinner and by Elwin Caler, one of Ms. Busby’s sons.  There is also an unknown profile from a bloodstain in the sons’ bedroom.  The unknown profiles may not be complete, but that is one of several issues that further testing might resolve.

My conclusions at this time are that the advisory paints an incomplete (therefore misleading) picture of the case and is overinterpreting both the previous DNA results and the present ones.  For example, the prosecution’s advisory summarized the testimony of a witness who claimed that Mr. Skinner threatened her, but the witness later recanted, a fact that was ignored.  The advisory failed to mention anything about two bloody handprints and bloody bootprints that have been discussed in one report on this case. "The State tested the bloody handprints against those of Hank Skinner.  Hank was good for the three handprints near the back of the house.  The handprint on the trash bag, and presumably the handprint on the front storm door, belonged to someone else." The existence of the jacket, the handprints that apparently do not match, and the bloody bootprints that are not his size are some of the objections to the prosecution's case.

With respect to DNA profiling the first problem is that the most compelling piece of exculpatory evidence, what one person identified as Donnell’s jacket, has been lost before it could be tested.  The jacket is stained with blood and sweat.  If either Donnell’s DNA were found on the jacket or if some of the blood were Twila’s), that would establish reasonable doubt, at the very least.  However, at present there is no reference profile of Robert Donnell (see also below).  The failure to obtain his profile could be a consequence of the investigatory tunnel vision that has plagued this case, as a previous entry here indicated.  I suggest that the State of Texas exhume his body or obtain his DNA in some other way.

A second problem centers around whether Twila was sexually assaulted on the night of her death.  The vaginal DNA profiling was performed by standard autosomal DNA methods, in which DNA deposited by a possible perpetrator can become overwhelmed by the victim’s own DNA.  There is nothing in the public record of Y-STR testing being performed, despite its routine use in sexual assaults.  The unique advantage of Y-STR profiling is that it involves only the Y chromosome, which women do not have; therefore, a male contribution to a mixture can be selectively amplified by the polymerase chain reaction.  Either the laboratory did not do Y-STR testing, or it did but the results were not included in the advisory.  In addition the fingernail clippings either showed Ms. Busby’s DNA, or they did not show any DNA.  Moreover, fingernail scrapings can also be tested with Y-STR forensics, which has marginally higher sensitivity than autosomal testing.  Mr. Skinner’s attorney indicated that the defense has requested additional testing; therefore, it is possible that Y-STR results will be forthcoming.

A third problem concerns the existence of DNA from at least one unknown person.  The evidence with respect to the knife appears inculpatory at first glance, yet it is undisputed that Mr. Skinner was bleeding that night.  It is likely that his contribution to the knife DNA profile arose from his blood, and one obvious possibility is that he cut himself while committing the murders. The defense should raise two alternate hypotheses.  There is chance that his profile arose from casual use of this knife around the house prior to the murder.  In addition, Mr. Skinner was incoherent according to one witness due to a combination of alcohol and codeine; therefore, his DNA could have been deposited by his possibly handling the knife at some point after the murder when he was allegedly stumbling around the house.  It is also undisputed that he was in close proximity to the victims that night; therefore, positing that he handled the knife, although speculative, is not unreasonable. 

The Skeptical Juror wrote, "Fresh blood drops were found on the sidewalk, near the front door of the house where the murders took place.  Those drops were tested.  The DNA from those drops belonged not to Hank Skinner, but to an unidentified male." The most recent DNA profiling also did not test the gauze, even though it may have been used to wipe the knife, which is odd.  I wonder whether this profile was similar to or different from the ones turned up in the most recent round of testing, and I also wonder who the DNA donor(s) is(are).  The former question might be addressed by treating the DNA from the blood drops on the sidewalk as something like a reference profile.  DNA mixtures are sometimes deconvoluted by a process known as sequential unmasking, in which a forensic worker is given information about the reference profiles in a gradual manner, only after completing a relevant portion of the analysis.  Yet even the mere existence of blood from another person calls for an explanation.  Was the blood fresh?  If so, who was bleeding besides Mr. Skinner, Ms. Busby, Mr. Caler, and Mr. Randy Busby, Twila’s other son (all of whose reference profiles are available)?

Finally, a fourth problem is that one of the samples may have been contaminated with some DNA from a laboratory worker.  This kind of event actually happens not infrequently in DNA profiling, but it does mean that the defense needs to scrutinize the negative controls and machine logs to look for other evidence of contamination.  Negative controls will detect large scale contamination that affects multiple items of evidence, but will often not detect sporadic contamination affecting a single sample, as Professor William Thompson has pointed out:  “However, the same processes that cause detectable errors in some cases can cause undetectable errors in others. If DNA from a suspect is accidentally transferred into a ‘blank’ control sample, it is obvious that something is wrong; if the suspect’s DNA is accidentally transferred into an evidentiary sample, the error is not obvious because there is another explanation — i.e., that the suspect is the source of the evidentiary DNA.”

One hopes and expects that future rounds of DNA testing will address some of these shortcomings.  Both the victims’ families and Hank Skinner deserve nothing less than a full accounting of what happened that terrible night.  However, the prosecutor brings charges in the name of the people; therefore, every citizen has some responsibility for what happens in a courtroom, including the times when there is a miscarriage of justice.